Pigs frequently are used in cleaning pipelines or separating product batches in a pipeline. In pipelines having internal obstructions, or a substantially varying diameter, there is significant risk in using a pig which may become stuck. And, such risk is enormous in deep water pipelines where the cost and lost time in recovering the stuck pig and re-opening the pipeline are substantial. For such pipelines, it is attractive to use a foam pig which is capable of changing size to get around obstructions or adapt to a different pipe diameter. However, foam pigs are not without problems also, and have been known to occasionally fail in the past. Foam pigs fail because (1) air or gas trapped in the foam compresses when the pig is subjected to pressure, decreasing its effective size and allowing a displacement fluid to bypass; (2) an open cell construction allows the displacement fluid to flow through the pig; and (3) a plastic coating on the pig to prevent bypass fails because the coating does not form a seal at the pipe wall.
Accordingly, the present invention is directed to overcoming the above-identified problems of the art by providing a novel pipeline pig which not only solves the above problems but also has other advantages as will become apparent hereinafter.